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I I I I I I I I I I I I I I I I I I I B-5 In our routing of streams through the reservoir, we have attempted to portray reservoir releases that would be made under operating conditions wherein the operators were able to anticipate, to a certain ex- tent, the timing of inflows, were attempting to min- imize the magnitude of releases to reduce impact on downstream properties, but yet still were, to a cer- tain extent, reacting to flows as they reached the reservoir. We allowed short term peaks in excess of 8280.3; we feel this is reasonable in dealing with 100yr type events. Further, snce determination of releases is based on judgement, extrame occurancy in reservoir routing is not appropriate, so our calcu- lations were done on a fairly rudimentary basis. 4. Concurrent Occurrence of Major Events: The general Granby Reservoir operating strategy is to fill the reservoir during June & July using snow- melt flows. Typical snow melt peak flows occur in the first half of June, but in 1983 the peak was de- layed to the end of June, and in 1984 there was an early peak in mid-May and a secondary peak in late June. In 1983 the reservoir reached 8279.5 in early July, and in 1984 in late June. In 1983, heavy rains hit the basin as the snow melt flows were re- ceding and the reservoir had just filled. Due to the reservoir strategy, it seems unlikely, but possible, that the peak snow melt flows would occur after the reservoir is filled. It is very likely that rain storms may occur after the reser- voir is filled and while there is still substantial snow melt flow. Theoretically, concurrent 100yr snow melt peaks and 100yr rainfall peaks have a probability of 1 in 10,000; concurrent 10yr peaks have a probability of 1 in 100. The theoretical probability of concurrent snow melt and rainfall peaks onto a full reservoir would be even more extreme. D. SELECTION OF MODEL EVENTS The intent of this study is to estimate flows at the Phase 2 site that have a minimum probability of 1 in 100, or a 100yr return event. Further, the intent is to be reasonably conservative, while still not providing unnecessarily high flow projections. with this intent, and based on the discussions above, we selected 3 events to evaluate, as follows: Case 1 - 100yr snow melt peak with reservoir full at elevation 8279.5. As stated above, this may not be a likely combination, but should be looked at.